Relatives of British soldiers killed in Iraq responded by heckling Blair, The Daily Telegraph reported.
"You lied. Your lies killed our son," called Rose Gentle, whose 19-year-old son Gordon, died there. "I hope you can live with it."
Blair's appearance before the panel headed by Sir John Chilcot was his second and last. He said a year ago, when he stood by his decision to join President Bush in the invasion, he gave the misleading impression he did not care about the casualties.
"I wanted to make it clear that, of course, I regret deeply and profoundly the loss of life, whether from our own armed forces, those of other nations, the civilians who helped people in Iraq or the Iraqis themselves," he said.
Family members responded with heckling and cries of "It's too late."
Relatives of British soldiers killed in Iraq made up about one-third of the audience. They were quiet for most of Blair's testimony but reacted angrily when he urged western countries to take on Iran.
"Stop trying to kill them then," one woman said, in tears, when Blair praised British soldiers.